The high affinity-receptor for IgE on mast cells and basophils plays a central role in immediate hypersensitivity reactions. Crosslinking of bound IgE by polyvalent antigen leads to aggregation of the receptors and cellular secretion of both preformed and newly synthesized mediators of inflammation. Although a large number of biochemical and morphological events have been observed to follow closely aggregation of the receptors, the molecular mechanism by which aggregation of the receptors generates these responses - a principal interest of our laboratory is still largely undefined. During the past year we have continued to employ mutated receptors in order to analyze which portions of the receptor participate in its principal functions. We continued to employ a three tiered strategy: 1) Construction of mutants and testing for their expression; 2) Identification of a suitable host cell which itself lacks receptors but which possesses the molecular machinery to respond to aggregation of transfected receptors in a manner characteristic of mast cells; 3) Assessment of which portions of the receptor are required to activate the molecules responsible for the cellular activation. Our new results show: 1) Expression of receptors is relatively tolerant of even drastic modifications of the cytoplasmic but not of the transmembrane domains. Some structural insights into the arrangement of the receptor subunits was obtained. 2) Some functional characteristics of the monomeric and aggregated receptor can be observed on easily transfectable COS-7 cells. 3) Other functional sequelae of receptor aggregation while not observed on the COS cells, but which we previously showed can be observed on a receptor-less mast cell line were characterized further. 4) Although the latter cell line will also express mutant receptors, accumulating a panel of such mutant transfectants may prove intolerably slow unless different approaches for selecting such mutant transfectants can be developed.